The prior art processes for the preparation of either PANOX (oxidized polyacrylonitrile) fibers or carbon fibers employs high heat and extended periods of time to obtain the final product. The final PANOX fiber or carbon fiber product in the prior art is often a flawed product that contains weak spots and micro-bubbles. This is because present processes do not control the rapid increase in heat when the fiber is heated above its fusion point. The prior art strategy increases the temperature of a PAN (polyacrylonitrile) fiber to just below its fusion point. Then the PAN fiber is SLOWLY heated for an extended period of time (approximately 3-4 hours). “Burn out” of the fiber is thus avoided. “Burn out” of the fiber occurs when the temperature of the fiber reaches the fusion point of the fiber. The internal temperature of the fiber then rapidly shoots up to 400 degrees C. and above. The result is that the fiber is destroyed. The fiber employed in prior art processes does not begin to cross-link until late in the heating cycle (near the fusion point of the fiber). Thus, the prior art methods depend on slowing down the heating process, and then slowly raising the temperature over the next couple of hours.
Prior art processes for preparing either PANOX fiber or carbon fiber include the use of about three or four monomers for preparing a polyacrylonitrile copolymer. The monomers can be: acrylonitrile (90-98% by wt.), itaconic acid (2-10% by weight), ethyl acrylate (2-5% by wt.), and vinyl sulfonic acid (1-3% by weight). In a first step, the four monomers are polymerized as by a precipitation polymerization process in the presence of a Redox catalyst. The resultant polyacrylonitrile copolymer is then wet spun into fibers. The fibers have a density of about 1.2 grams/cc. The PAN (polyacrylonitrile) fibers are then removed to a first oxidation zone. The fibers are SLOWLY heated in an energy intensive process for a time of about 3-4 hours. The process is carefully monitored to prevent runaway heating from the interior of the fibers. This process is not cost effective because of the extended heating cycle.
PANOX (oxidized polyacrylonitrile) fibers are withdrawn from the first oxidation zone. The density of the PANOX fibers is about 1.4 grams/cc. The PANOX fibers can be used in areas of technology such as sporting equipment and the like. If pristine carbon fibers are desired, the PANOX fibers are removed to a second oxidation zone which is a high temperature carbonization unit. Temperatures in the carbonization unit are from 1000-2000 degrees C., or even higher. All atoms other than the carbon atoms are vaporized away from the fibers to obtain a pristine carbon fiber.
U.S. Pat. No. 5,462,799 discloses the preparation of a carbon fiber wherein a precursor PAN-fiber is oxidized, carbonized and if necessary graphitized to make the carbon fiber.
U.S. Pat. No. 5,281,477 discloses the preparation of a carbon fiber having high tenacity and high modulus of elasticity. Pretreated fibers are passed through a series of three carbonization zones.
The problems of the current methodology for preparation of carbon fiber are as follows: (1) failure to recognize the fact that amidines are the true initiators in the preparation of carbon fiber; (2) failure to recognize that metal ions “poison” the fiber at every step of the process; and (3) failure to recognize that the first oxidation step can be performed in a substantially reduced time frame.
Therefore there is a need for a process of preparing superior PANOX fiber or carbon fiber that overcomes the drawbacks of the prior art.